It is known by those skilled in the art that in many instances metal fluorides may be produced by combining a metal or a metal compound (e.g. metal salts) with hydrofluoric acid. Wherein the metal compound is a metal chloride, the reactions are basically as follows:MCl+HF→MF+HCl↑MCl2+2HF→MF2+2HCl↑MCl3+3HF→MF3+3HCl↑MCl4+4HF→MF4+4HCl↑
The reaction between metal chlorides and hydrofluoric acid may be endothermic. In such cases, in order for the reactions to go to completion, the reactants must absorb heat from their environment. Where the reactions are endothermic, it has been observed that the greater the rate of heat delivery to the reactants and the higher the temperature at the time of the reaction, the smaller are the resultant metal fluoride particles. In general, the smaller the metal fluoride particles, the greater is the exposed surface area per unit of weight of the resultant metal fluoride. In view of the fact that catalysts are surface active agents, the greater surface area per unit of weight commonly associated with the smaller particles would be expected to exhibit greater catalytic activity, and such has been shown to be the case.
In the event the reaction between the metal chloride and the anhydrous hydrofluoric acid is exothermic, the reaction would need to give up heat in order to go to completion. In which case the conclusion cited above would be expected to be the opposite for the exothermic reaction case.
The process of combining metal chlorides and hydrofluoric acid may have somewhat varied results depending on which metal chloride is employed; however, in order to illustrate the general reaction, the specific case of the manufacture of ferric trifluoride by combining ferric trichloride and anhydrous hydrofluoric acid is cited herein.
The process of combining ferric trichloride and anhydrous hydrofluoric acid causes the following events to occur:    The ferric trichloride is dissolved and ionized in the liquid anhydrous hydrofluoric acid.    The individual molecule of dissolved and ionized ferric trichloride exchanges the first chloride atom with a fluoride atom from the liquid, ionized anhydrous hydrofluoric acid source and in so doing, the individual molecular reaction product remains soluble and ionized as FeFCl2, ferric fluoride dichloride along with the evolution of hydrochloric acid gas (at one atmosphere and at temperatures above −84.9° C.).    The individual dissolved and ionized ferric fluoride dichloride exchanges the second and third chloride atoms with two fluoride atoms from the liquid ionized anhydrous hydrofluoric acid source and, in so doing, the ferric trifluoride molecule thus formed becomes insoluble in the liquid anhydrous hydrofluoric acid and precipitates as a lime green solid, while simultaneously liberating additional hydrochloric acid gas.
It is the currently accepted practice to add liquid anhydrous hydrofluoric acid to solid ferric trichloride when manufacturing ferric trifluoride, as is set forth, for example, in detail in U.S. Pat. No. 4,938,945, the disclosure of which is hereby incorporated by reference herein in its entirety. This method is practiced for various reasons cited in U.S. Pat. No. 4,938,945. An additional reason for adding the liquid anhydrous hydrofluoric acid to solid ferric trichloride is safety. It is generally accepted that there is a lower propensity for the reactants to splatter and, hence, this method is deemed to be a safer process than adding the ferric trichloride to the anhydrous hydrofluoric acid. However, it is manifestly obvious that, in this process, the first weight aliquot of ferric trichloride is exposed to a very limited quantity of anhydrous hydrofluoric acid (very low weight ratio of anhydrous hydrofluoric acid to ferric trichloride). Each subsequent aliquot of ferric trichloride is also exposed to a limited weight ratio of anhydrous hydrofluoric acid to ferric trichloride but eventually sufficient anhydrous hydrofluoric acid is added until the perceived optimum weight ratio is established. However, at that point in time all of the ferric trichloride has reacted at a weight ratio of anhydrous hydrofluoric acid to ferric trichloride that is significantly below the optimum level. It is contended that this aspect of the process results in relatively large primary particles, the agglomeration of the primary particles, slow reaction times, incomplete reactions, low to no catalytic activity, and poor quality control with respect to the chemical and physical characteristics of the resultant ferric trifluoride product.
Furthermore, it is the currently accepted practice to combine the ingredients in the ferric trifluoride manufacturing process at atmospheric pressure. In light of the fact that liquid anhydrous hydrofluoric acid boils at 19.8° C. (67.6° F.) at standard atmospheric pressure, the boiling point of the anhydrous hydrofluoric acid limits the temperature to which the environment of the reactants may be elevated prior to and during the reaction. In the absence of temperature controlling apparatus and/or equipment, the prior art manufacturing process will tend to experience cooling while the ingredients are being combined because of the endothermic nature of the reaction. Thereafter, the resultant product will tend to modulate at ambient temperature or at 19.8° C. (67.6° F.), the boiling point of anhydrous hydrofluoric acid at standard atmospheric pressure, if the ambient temperature is greater than 19.8° C. (67.6° F.), once the reaction is complete.
Following the completion of the reaction, it is currently accepted practice to allow the resultant ferric trifluoride product to remain submerged in the liquid anhydrous hydrofluoric acid three to ten days. The longer residence time (“pickling time”) generally results in a more complete reaction which in turn yields a more pure ferric trifluoride product.
Following the reaction period and the residence period, it is currently accepted practice to separate the resultant solid ferric trifluoride product from the remaining anhydrous hydrofluoric acid by decanting and/or evaporating the acid. Thereafter, the ferric trifluoride product is dried in stages until the ultimate temperature is approximately 250° F. In so doing, it is anticipated that any free residual anhydrous hydrofluoric acid and/or any free water shall be driven off, leaving an anhydrous ferric fluoride product. The product is then packaged in such a manner as to isolate it from the environment and avoid the absorption of moisture, etc.